The use of optical fibers in communications is growing at an unprecedented rate. Low loss optical fibers which are produced by any one of several techniques may be assembled into ribbons which are then assembled into cables, or stranded into cables, or they may be enclosed singularly in a jacket and used in various ways in a central office, for example.
In order to assure that the low loss fibers which are produced today are not diminished in their effectiveness in systems, the fibers must be connected through intermateable connectors which preserve those low losses. For fiber ribbons, connectors comprise grooved chips which hold a plurality of fibers of one ribbon in alignment with fibers of another ribbon. Such a connector is shown for example in U.S. Pat. No. 3,864,018 which issued on Feb. 4, 1975 in the name of C. M. Miller.
For single fiber cables, connections may be made through a connector which is referred to as a biconic connector. See U.S. Pat. Nos. 4,107,242 and 4,512,630 which issued on Aug. 15, 1978 and Apr. 23, 1985, in the name of P. K. Runge. That connector includes a housing in which is mounted a biconic alignment sleeve. The sleeve includes two truncated, conically shaped cavities which communicate with each other through a common plane which has the least diameter of each cavity. Each of two fibers to be connected is terminated with a plug comprising a primary pedestal or truncated, conically shaped end portion which is adapted to be received in one of the cavities of the sleeve. The conically shaped surfaces of the plug and of the sleeve serve as alignment surfaces. The fiber extends through a passageway in the plug and has an end which terminates in a secondary pedestal of the plug. Generally a plug is molded about an end portion of an optical fiber; however, there is a demand for plugs having passageways for receiving optical fibers in the field. A cylindrically shaped portion of the plug is connected to the truncated end. The plug is urged into seated engagement with the wall defining the cavity in which it is received.
Minimal loss between the connected fibers is achieved when the cores of fibers which are terminated by the plugs are aligned coaxially and when the longitudinal offset along the axes of the plugs is zero and fiber end faces, each of which is planar, contact in a common plane. Considering the size of the fibers, for example one with a core diameter of 8 microns and a cladding diameter of 125 microns, the task of providing conical plug and sleeve surfaces in order to meet alignment and end separation requirements is a formidable one. Generally, the plugs are molded from a transfer molding grade epoxy composition material. Although the surface tolerances which are achieved when molding the alignment sleeves and conic tapers are excellent, they are not sufficient to achieve consistently the desired alignment and end separation.
Problems arise because the opening in the end face of the pedestal and hence the fiber core may not be centered with respect to the axis of revolution of the conically shaped surface of the plug. The axis of revolution of a conically shaped end portion also may be referred to as its conical axis. As a result, the cores of the fibers terminated by two plugs held in the sleeve may have sufficient transverse offset to affect adversely transmission of signals.
The flow of molten material during a molding process which is used to manufacture biconic connectors causes the optical fiber end portion about which a plug is molded or a passageway which is adapted to receive an optical fiber to be disposed along an axis which may not be parallel to the axis of the plug. The angle between the fiber axis and the axis of revolution is commonly referred to as the "exit angle" of the connector. Consequently, the light emitted from one optical fiber may not be parallel to the axis of the receiving fiber. This problem is referred to as angular offset.
Control of the exit angle is essential for achieving low loss connections and high yields in single mode connector manufacture. This has been achieved by methods and apparatus which are disclosed and claimed in commonly assigned, U.S. Pat. No. 4,721,357 which was filed of even date herewith in the names of J. Kovalchick, J. Mark Palmquist, and R. Treder. Control of this parameter is necessary so that when two plugs are disposed in an alignment sleeve, not only will the end faces just touch, but the fiber axes will be coaxial.
Seemingly, the prior art is devoid of a simple solution to the problem of providing production plugs at a relatively high yield for biconic connectors which may be used for multi or single mode lightguide fibers. Each production plug must be such that a centroid of the core of an optical fiber terminated therein in an end face of the plug is coincident with the axis of revolution of the truncated, conically shaped surface of the plug. Desirably, the solution does not require additional elements or time in the connection procedures, but instead involves an automatic adjustment of molded plugs to achieve precision without the need of a skilled machinist. What is needed are methods and apparatus for measuring the exit angle and correcting for transverse offset of the optical fibers from the axis of revolution of the end portion of the plug and reconfiguring a new end portion having an axis which is coincident with the centroid of the fiber core or the passageway at an end face of the fiber.